An increasing number of lighting systems employ light emitting diodes (LED or LEDs) because of their small size, low power requirements and general lack of excessive heat generation during operation. Many single color light emitting diodes are available; however, in the area of green emission there exists the so-called “green gap” where optimum color efficiency is not available. Therefore, it would be an advance in the art to fill this “green gap” with a light source that is convenient and economical to manufacture.
When it is desired to construct an LED light source that produces a color different from the output color of the LED, it is known to convert the LED light output having a peak wavelength (the “primary light”) to light having a different peak wavelength (the “secondary light”).
The conversion process involves absorbing the primary light by a wavelength-converting material such as a phosphor or mixture of phosphors thereby exciting the phosphor material, which emits the secondary light. The peak wavelength of the secondary light depends on the type of phosphor material, which can be chosen to provide secondary light having a particular peak wavelength. This process may be generally referred to as “wavelength conversion” and an LED combined with a wavelength-converting material, such as phosphor, to produce secondary light, may be described as a phosphor conversion LED (pc-LED).
In a known configuration, an LED die, such as a III-V nitride die, is positioned in a reflector cup package and a volume, conformal layer or a thin film including wavelength-converting material is deposited directly on the surface of the die. In another known configuration, the wavelength-converting material may be provided in a solid, self-supporting flat plate, such as a ceramic plate, single crystal plate, or thin film structure. The plate may be attached to the LED, e.g. by wafer bonding, sintering, gluing, etc. Such a plate may be referred to herein as a “wavelength-converting plate.” Another known approach is to provide the wavelength-converting material in a solid, self-supporting dome formed separately from the LED and attached thereto.